This is a type of
handlebar commonly seen on newer
mountain bikes. Instead of the older
flat bars (which were often seen with
bar ends), riser bars are bowed upwards and slightly back to give the rider a higher, wider sitting position on the bike. Coupled with a shorter
stem, this changes the
geometry of the bike somewhat, making it more suitable for
technical trails and downhills.
Climbing becomes a bit more difficult, however, as you aren't as far forward on the bike anymore.
Many different riser bars are around, the main differences being the degree of curvature in them. Most are made from aluminum, while some high-end ones are made from a carbon fibre composite to save weight.